diff --git a/src/Init/Core.lean b/src/Init/Core.lean
index 945cca2b6a..e0dcee71ed 100644
--- a/src/Init/Core.lean
+++ b/src/Init/Core.lean
@@ -436,11 +436,14 @@ def std.prec.maxPlus : Nat := std.prec.max + 10
 infixr `×` := Prod
 -- notation for n-ary tuples
 
-/- Some type that is not a scalar value in our runtime.
-   TODO: mark opaque -/
+/- Some type that is not a scalar value in our runtime. -/
 structure NonScalar :=
 (val : Nat)
 
+/- Some type that is not a scalar value in our runtime and is universe polymorphic. -/
+inductive PNonScalar : Type u
+| mk (v : Nat) : PNonScalar
+
 /- For numeric literals notation -/
 class HasOfNat (α : Type u) :=
 (ofNat : Nat → α)
@@ -1098,6 +1101,8 @@ instance : Inhabited Nat := ⟨0⟩
 
 instance : Inhabited NonScalar := ⟨⟨arbitrary _⟩⟩
 
+instance : Inhabited PNonScalar.{u} := ⟨⟨arbitrary _⟩⟩
+
 instance : Inhabited PointedType := ⟨{type := PUnit, val := ⟨⟩}⟩
 
 class inductive Nonempty (α : Sort u) : Prop
diff --git a/src/Init/Data/Array/Basic.lean b/src/Init/Data/Array/Basic.lean
index 1922e4012a..55781c0736 100644
--- a/src/Init/Data/Array/Basic.lean
+++ b/src/Init/Data/Array/Basic.lean
@@ -445,21 +445,21 @@ section
 variables {m : Type u → Type w} [Monad m]
 variable {β : Type u}
 
-@[specialize] unsafe partial def umapMAux (f : Nat → α → m β) : Nat → Array α → m (Array β)
+@[specialize] unsafe partial def umapMAux (f : Nat → α → m β) : Nat → Array (PNonScalar.{u}) → m (Array (PNonScalar.{u}))
 | i, a =>
   if h : i < a.size then
      let idx : Fin a.size := ⟨i, h⟩;
-     let v   : α          := a.get idx;
-     let a                := a.set idx (unsafeCast ());
-     do newV ← f i v; umapMAux (i+1) (a.set idx (unsafeCast newV))
+     let v   : PNonScalar := a.get idx;
+     let a                := a.set idx (arbitrary _);
+     do newV ← f i (unsafeCast v); umapMAux (i+1) (a.set idx (unsafeCast newV))
   else
-     pure (unsafeCast a)
+     pure a
 
 @[inline] unsafe partial def umapM (f : α → m β) (as : Array α) : m (Array β) :=
-umapMAux (fun i a => f a) 0 as
+@unsafeCast (m (Array PNonScalar.{u})) (m (Array β)) $ umapMAux (fun i a => f a) 0 (unsafeCast as)
 
 @[inline] unsafe partial def umapIdxM (f : Nat → α → m β) (as : Array α) : m (Array β) :=
-umapMAux f 0 as
+@unsafeCast (m (Array PNonScalar.{u})) (m (Array β)) $ umapMAux f 0 (unsafeCast as)
 
 @[implementedBy Array.umapM] def mapM (f : α → m β) (as : Array α) : m (Array β) :=
 as.foldlM (fun bs a => do b ← f a; pure (bs.push b)) (mkEmpty as.size)
diff --git a/tests/lean/run/125.lean b/tests/lean/run/125.lean
new file mode 100644
index 0000000000..d9638ef7c8
--- /dev/null
+++ b/tests/lean/run/125.lean
@@ -0,0 +1,28 @@
+class HasElems (α : Type) : Type := (elems : Array α)
+def elems (α : Type) [HasElems α] : Array α := HasElems.elems α
+
+inductive Foo : Type
+| mk1 : Bool → Foo
+| mk2 : Bool → Foo
+
+open Foo
+
+instance BoolElems : HasElems Bool := ⟨#[false, true]⟩
+instance FooElems  : HasElems Foo  := ⟨(elems Bool).map mk1 ++ (elems Bool).map mk2⟩
+
+def fooRepr (foo : Foo) :=
+match foo with
+| mk1 b => "OH " ++ toString b
+| mk2 b => "DR " ++ toString b
+
+instance : HasRepr Foo := ⟨fooRepr⟩
+
+#eval elems Foo
+
+#eval #[false, true].map Foo.mk1
+
+def Foo.toBool : Foo → Bool
+| Foo.mk1 b => b
+| Foo.mk2 b => b
+
+#eval #[Foo.mk1 false, Foo.mk2 true].map Foo.toBool